Motor with no load

drbitboy said:

Alternate method to detect cavitation or empty inlet/sump, possibly preferable to adding a level switch?

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OK, that's valid, except the pump itself represents SOME load, as opposed to an uncoupled motor. And for this, all you need to do is run it unloaded, measure than value, and set up a detection method using that value as a trigger. It doesn't really matter what that value is as a percent of motor FLC. But to be honest, a BETTER way to do that is to use a device that measures WATTS, it's more exact with regard to actual shaft power. Lots of "smart overloads" now offer this as an option, I use it extensively.


One thing I have been doing with this is to monitor the motor shaft kW with the OL relay, then compare that to a pump flow transducer signal. Flow and kW should track together consistently, so I want to see if they start to diverge. If the kW increases disproportionately to the flow, that means bearings are wearing out. If the kW decreases disproportionately to flow, there is cavitation, blockage or air getting into the system. Either way, I want to know about it.

jraef said:

Flow and kW should track together consistently

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interesting diagnostic tool; I have seen businesses that advertise this kind of failure prediction, but if a version, perhaps more coarse, is available via a few lines of code/ladder for lower cost, that has to be a win.





Flow x deltaP will always track*, so in your case when kW rises but not flow, that is an indirect pressure measurement.


* assuming constant friction, as you note in the case where kW rises but not flow.


Actually, either case could be a change in deltaP, but that is probably worth investigating as well (blocked pipe; stuck valve, etc.).


OT but related:


My dad generated business for GE by running valves-wide-open tests at fixed inlet conditions on large- and medium-sized turbines, essentially treating the turbine as a flow element and calculating its flow coefficient. Variation in coefficient over meant the process has changed i.e. degraded, which meant lower efficiency and a corresponding higher heat rate (Btu/kWh). That loss in heat rate represented increased operating costs that could be recovered for the capital cost of having GE re-shape and/or re-build the blades.


ROI: it's what's for dinner.